Recall that surrounding any object with mass, or collection of objects with mass, is agravitational field. Any mass placed in a gravitational field will experience agravitational force. We defined the field strength as thegravitational force per unitmasson any “test mass” placed in the field:

g

=

F

/

m

.

g

is a vector that points inthe direction of the net gravitational force; its units are N

/

kg.

F

is the vector forceon the test mass, and

m

is the test mass, a scalar.

g

and

F

are always parallel. Thestrength of the field is independent of the test mass. For example, near Earth’ssurface

mg

/

m

=

g =

9.8 N

/

kg

for any mass. Some fields are uniform (parallel,equally spaced fields lines). Nonuniform fields are stronger where the field lines arecloser together.

Earth

Earth’s surface

10 kg98 N

mF

uniform field nonuniform field

Electric Fields: Intro

Surrounding any object with charge, or collection of objects with charge, is aelectric field. Any charge placed in an electric field will experience a electricalforce. We defined the field strength as theelectric force per unit chargeon any “testcharge” placed in the field:

E

=

F

/

q

.

E

is a vector that points, by definition, in thedirection of the net electric force on a positive charge; its units are N

/

C.

F

is thevector force on the test charge, and

q

is the test charge, a scalar.

E

and

F

are only parallel if the test charge is positive. Some fields are uniform (parallel, equallyspaced fields lines) such as the field on the left formed by a sheet of negative charge. Nonuniform fields are stronger where the field lines are closer together, such as thefield on the right produced by a sphere of negative charge.